Daily Mortality Rates of Larval and Juvenile American Shad (Alosa sapidissima) in the Connecticut River with Changes in Year-Class Strength

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Abstract
Age-specific growth and survival rates were estimated for larval and juvenile American shad (Alosa sapidissima) from the Connecticut River from 1979 to 1982. Relative indices (CPE) of year-class strength of juveniles determined from seine sampling in 1978–82 and 1966–73 are compared with resulting levels of adult recruitment. Length–age data for larval and juvenile American shad were determined from sagittal otoliths. Length increments among larval and juvenile shad followed an asymptotic pattern with age, both life stages being well described by the Gompertz equation. Larval survivorship curves from 1979 to 1982 were age specific, with mortality rates of 19.8–25.6%/d for first feeding larvae and 4.3–8.7%/d for larvae approaching metamorphosis. By contrast, juvenile mortality rates were much lower (1.8–2.0%/d) and more consistent among years. Juvenile indices of year-class strength from 1966 to 1973 were positively correlated (r = +0.92, df 7, P < 0.001) with recruitment levels of adult females 4–6 yr later, suggesting that year-class strength of shad is established prior to the juvenile stage.

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